Single transmembrane domain insulin-like growth factor-II receptor and G protein: potential interaction and its significance in brain function

胰岛素样生长因子-II（IGF-II）受体是一种单跨膜糖蛋白，含有一个大的胞外结构域和小的胞质尾区。该受体通过两个不同的位点与IGF-II和携带甘露糖6-磷酸的溶酶体酶相互作用。大多数受体在反式高尔基体网络/内体区室中表达，在那里它们将新合成的溶酶体酶从分泌途径转移到内体和溶酶体。位于质膜上的受体亚群调节分泌的溶酶体酶的内吞作用，介导IGF-II的内化和降解。与细胞内运输不同，IGF-II受体在IGF-II跨膜信号传导中的作用仍然知之甚少。我们最近报道，IGF-II受体的大脑是耦合到G蛋白和IGF-II或Leu 27 IGF-II，一种类似物，优先结合IGF-II受体的激活，增强乙酰胆碱的释放，但减少GABA释放从大鼠海马通过G蛋白敏感的途径。此外，我们观察到IGF-II受体与G蛋白偶联受体相互作用蛋白β-arrestin-2相关，并在激动剂刺激后从洗涤剂抗性膜转移到洗涤剂可溶性膜组分。这些结果使我们假设，在大脑中的单个跨膜结构域IGF-II受体耦合到G蛋白，其激活可以通过触发G蛋白连接的细胞内信号传导机制介导特定的生物学反应。为了解决这个问题，我们提出i）建立IGF-II受体与G蛋白/β-抑制蛋白-2相互作用的特异性，ii）评估IGF-II受体是否直接或间接与G蛋白相互作用，iii）确定IGF-II减弱海马GABA释放的机制，和iv）研究IGF-II小鼠脑中GABA能和胆碱能系统的改变。II受体过表达转基因小鼠。我们相信，这项研究不仅可以为IGF-II受体在正常大脑中的生理相关性提供证据，而且还将为更好地了解其在神经退行性疾病中的作用铺平道路。

The insulin-like growth factor-II (IGF-II) receptor is a single transmembrane glycoprotein containing a large extracellular domain and small cytoplasmic tail. The receptor interacts with the IGF-II and mannose 6-phosphate-bearing lysosomal enzymes via two distinct sites. A majority of the receptors are expressed within trans-Golgi network/endosomal compartments, where they divert newly synthesized lysosomal enzymes from the secretory pathway to endosomes and lysosomes. A subset of receptors located on the plasma membrane regulates endocytosis of secreted lysosomal enzymes, mediates internalization and degradation of IGF-II. Unlike intracellular trafficking, the role of the IGF-II receptor in the transmembrane signaling of IGF-II remains poorly understood. We have recently reported that the IGF-II receptor of the brain is coupled to a G protein and its activation by IGF-II or Leu27IGF-II, an analog that binds preferentially to the IGF-II receptor, potentiates acetylcholine release but decrease GABA release from the rat hippocampus by a G protein-sensitive pathway. Additionally, we observed that IGF-II receptors are associated with G protein-coupled receptor interacting protein beta-arrestin-2 and are translocated from detergent-resistant membrane to detergent-soluble membrane fractions following stimulation with agonist. These results led us to hypothesize that the single transmembrane domain IGF-II receptor in the brain is coupled to a G-protein and its activation can mediate specific biological responses by triggering G protein-linked intracellular signaling mechanism. To address this issue we propose i) to establish the specificity of the IGF-II receptor interaction with G protein/beta-arrestin-2, ii) to evaluate whether the IGF-II receptor interacts directly or indirectly with the G protein, iii) to determine the mechanisms by which IGF-II attenuates hippocampal GABA release and iv) to study alterations in the GABAergic and cholinergic systems in the brain of IGF-II receptor overexpressing transgenic mice. We believe that this study should not only provide evidence for the physiological relevance of IGF-II receptor in normal brain but it will also pave the way for a better understanding of its role in neurodegenerative diseases.